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Volume 66, 1937
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Notes on Hydrogen-Ion Concentration of Forest Soils in the Vicinity of Dunedin, New Zealand

[Read before the Otago Branch, November, 1935; received by the Editor, January 28, 1936; issued separately, September, 1936.]

During the year 1931 and later, when studying the natural regeneration of the indigenous forests in the neighbourhood of Dunedin, the authors paid some attention to the hydrogen-ion concentration of the forest soils in the area, and this paper embodies certain features of interest which have come to light during these investigations.

Most of the p H values were determined in the field, and all determinations were made by colorimetric methods, using either the B.D.H. Capillator or Comparator. The usual practice followed was to take samples of soil from any one spot at-successive depths from the surface of 0–8 cm., 8–16 cm., and 16–24 cm. In many cases samples were also taken from greater depths, but as the values of these did not as a rule alter to any great extent from those of the soils immediately above, this practice was not continued.

Nothofagus Menziesii Communities.

Investigations were first directed to the examination of soils occurring in the various associations of silver southern-beech (Nothofagus Menziesii) which exist in the area. These soils were without exception strongly acid in reaction and exhibited remarkable uniformity in this respect, the surface layers having in most cases the lowest p H values (highest acidity). Altogether some 60 determinations were made, and the results tabulated below may be taken as a fair average for this class of forest in the area.

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Table A—(Nothofagus Menziesii Associations).
Locality. PH at different depths.
0–8 cm. 8–16 cm. 16–24 cm.
Boulder Hill 4.4 4.8 5.0
Bethune Gully 4.6 4.6 5.2
Silver Peaks (east) 4.4 4.4 4.6
Silver Peaks (north) 4.2 4.2 4.2
Silver Peaks (west) 4.2 4.2 4.4
Ferguson Creek 4.4 4.4 4.4
Maungatua (south) 4.6 4.6 4.8
Maungatua (north) 4.8 4.8 5.2

Determinations made in several Nothofagus communities elsewhere showed similar results.

Within these local Nothofagus communities N. Menziesii is usually the only tall tree present, though in several of the groups isolated specimens of Libocedrus Bidwillii occur. Trees common to the second layer are Griselinia littoralis, Nothopanax Colensoi, Carpodetus serratus, Coprosma linariifolia, Pittosporum tenuifolium,

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Pseudopanax crassifolium, Suttonia australis, Myrtus pedunculata and Wintera colorata. Also present in most of the communities are Myrtus obcordata, Nothopanax Edgerleyi and N. simplex. The shrublayer is composed of young plants of the foregoing species together with Coprosma foetidissima, C. propinqua, C. rhamnoides and C. rotundifolia. Seedling or juvenile plants of Nothofagus are extremely scarce and tree-ferns are generally absent or few in number, though the prostrate Alsophila Colensoi is sometimes present. Ferns that are common as epiphytes are Asplenium flaccidum, Polypodium diversifolium and Cyclophorus serpens, this last ascending the trees to the topmost branches. The principal ground-species are Blechnum discolor (often dominant), B. procerum, B. fluviatile, B. lanceolatum, Polystichum vestitum, and Astelia nervossa. Rubus australis, Parsonsia heterophylla and Clematis indivisa occur occasionally, and in most of the localities lichens, mosses and liverworts are much in evidence.

The conditions met with on the forest-floor in all the above associations are similar, a thick undecomposed layer of fallen leaves, a mass of interlaced rootlets very near the surface, and regeneration of Nothofagus practically non-existent under the canopy of the associations. The formation of the leaf-litter is probably due to the dense shade depressing the temperature of the forest-floor, thus slowing down the biological and chemical processes in the soil and eventually leading to humus accumulation. It must be remembered in this connection that leaves of different species vary greatly in their power to resist decomposition. What effect the high soil acidity in itself has on the regeneration of seedling beech is difficult to estimate without direct experiment. Other factors, such as lack of light, leaf-litter and humus-accumulation, certainly play a part, and it is interesting to note that investigations by Hesselman on the conifers in North Sweden show that seedlings appeared and became established there only where the soils were nitrifying.

It is probable that the nature of the original rock from which the soil has been derived has some effect on its hydrogen-ion concentration; but as far as our observations go, once forest or other plant-formations have been in existence on any area for long periods of time, the nature of the underlying rock generally does not seem to have any great effect on the surface soil-reaction, old established forest, for instance, making in the main its own soil conditions.

It is noteworthy that lack of regeneration in N. Menziesii forests is associated with poor light conditions, accumulation of leaf-litter and marked acidity of the soil, while regeneration takes place where there is an abundance of light, absence of leaf-litter and where the soil is, in the majority of cases, much nearer neutrality.

Rain-forest Communities.

The next class examined were soils of the various rain-forest associations present in the botanical area. These, unlike the Nothofagus communities, showed no uniformity, but were characterized by values that fluctuated in a remarkable manner, wide differences in hydrogen-ion concentration being observed in quite small areas;

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in general, also, they proved to be considerably less acid than the soils of the Nothofagus groups, some indeed being neutral or even slightly alkaline.

Table B shows some instances of the diversity in p H values met with in rain-forest communities of the area.

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Table B—(Rain-forest Communities).
Species from the vicinity of which soil was taken. Locality. PH at different depths
0–8 cm. 8–16 cm. 16–24 cm.
Large-leaved Dicotyledons
Melicytus ramiflorus Mt. Flagstaff 7.2 7.2 7.2
" Halfway Bush 7.4 7.4 7.4
" Mihiwaka 7.2 7.0 7.0
" " 7.2 6.8 6.8
Fuchsia excorticata Mt. Flagstaff 7.2 7.2 7.2
" " 7.4 7.2 6.8
Griselinia littoralis " 7.2 7.2 7.0
" Halfway Bush 7.4 7.2 7.2
Myoporum laetum (coastal) Waikouaiti 7.2 7.2 7.0
" " 7.0 7.0 7.0
Average value = 7.2 7.2 7.1

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Species from the vicinity of which soil was taken. Locality. PH at different depths
0–8 cm. 8–16 cm. 16–24 cm.
Dacrydium cupressinum Mt. Flagstaff 5.4 5.0 5.0
" " 5.2 5.4 5.6
" " 5.2 5.4 5.4
" " 5.0 5.0 5.2
" Mihiwaka 5.0 5.2 5.2
" Taieri Mouth 5.0 5.0 5.2
" " 6.2 6.0 6.0
" Halfway Bush 5.6 5.4 5.0
" Double Hill 5.0 5.2 5.2
" Leith Valley 5.2 5.4 5.4
" Waitati 4.2 4.4 4.4
Podocarpus ferrugineus Mt. Flagstaff 5.6 5.6 5.8
" Mihiwaka 4.8 5.0 5.0
" Taieri Mouth 6.2 6.2 6.0
Podocarpus dacrydioides Henley 6.2 5.8
Podocarpus spicatus " 6.2 5.8 5.2
Podocarpus Hallii Mt. Flagstaff 7.0 7.0 7.0
Podocarpus totara " 7.0 7.0 7.0
" " 6.4 6.4 6.4
" " 7.2 6.8 6.8
" Henley 6.8 6.0
" Mihiwaka 5.2 5.2 5.2
Average value = 5.7 5.6 5.6
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Such differences in value necessitated further study over the whole area and led to the conclusion that each species definitely influences the reaction of the soil in its immediate vicinity. Readings showing wide divergences in p H value can be obtained from under trees only a few metres apart, the degree of difference depending upon the species from under which the soils are taken. In order that the maximum effect of a tree's influence upon the soil beneath it may be more or less accurately gauged it is necessary, however, to collect the soils from under large mature trees where ground-plants are absent and in situations where the soil-reaction caused by one species cannot unduly affect that of another. It is advisable also to avoid localities where wind may constantly remove the leaf-litter from under one species and transfer it to another.

The distinct effect on soil reaction by the individual can be observed in any mixed forest in the area. For example, on Mt. Mihiwaka soils were taken from under specimens of Dacrydium cupressinum and Melicytus ramiflorus situated in that forest within 5 m. of one another; the D. cupressinum value was p H 5.0 and that of M. ramiflorus was p H 7.2. Again, on Mt. Flagstaff the values for the soils under single specimens of Pseudopanax crassifolium and Griselinia littoralis growing within 3 m. of one another were respectively p H 5.4 and p H 7.0. Also, near Double Hill the values for individuals of D. cupressinum and Podocarpus totara occurring 6 m. apart were p H 5.0 and p H 7.4. These are not isolated instances, and equally great differences in p H values are the rule and not the exception in forests containing a number of species. A very striking example was observed in a M. ramiflorus association occurring on the eastern slopes of Mt. Flagstaff in which a few scattered mature specimens of D. cupressinum still exist; the soil as a whole was near the neutral point, but readings taken under these widely-separated D. cupressinum trees gave markedly acid values all in the vicinity of p H 5.0.

The effect on the soil of two species recently planted in neighbouring plots in the garden of one of the authors may be quoted. The plots, each about 6 m. in diameter and 2 m. apart, were planted in September, 1923, with six young Nothofagus and P. totara trees respectively. The plots were similar in all respects, having the same aspect, drainage and exposure and presumably having the same p H values. Their soil reactions were first taken in August, 1932, the Nothofagus soil then having a p H value of 4.7 while the totara soil gave p H 4.9. In November, 1935, the soils were again examined, the beech plot showing p H 4.4 and the totara p H 5.0. The difference between the hydrogen-ion concentration of the soils of the two plots nine years after planting (1923 to 1932) amounted therefore to 0.2 units of p H value, and in 1935, a period of three years only, this difference had increased to 0.6 units. Electrometric determinations were also made, the final figures for the beech and totara plots by this method being p H 4.36 and p H 5.04 respectively, showing a difference in p H value of 0.68 units, a figure agreeing very closely with that obtained colorimetrically. It is interesting to note that in the comparatively short time of 12

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years the Nothofagus plot is covered with a dense mat of undecomposed leaves in which no weed species grow, while the totara soil is practically bare of litter and requires weeding at intervals.

Average Values for Soils under Certain Rain-forest Species.

An endeavour was made to obtain values for some rain-forest species. Melicytus ramiflorus, Podocarpus totara and Dacrydium cupressinum were selected as appearing likely—from preliminary determinations—to promote neutrality, mild acidity and pronounced acidity respectively. Over 50 soils from under mature specimens of M. ramiflorus were collected from different parts of the area and examined, the average p H value of these being 7.0. Twenty-six soils from beneath P. totara trees gave an average of p H 6.4; the values of these soils varied widely, ranging from p H 5.2 to p H 7.6, though the majority were near neutrality. That P. totara thrives on soils at or near neutrality can be shown by the presence of almost pure stands of this species growing on the upper slopes of the Chalk Range (Inland Kaikoura Mts.), where the parent rock is only a short distance from the surface; in addition, quite a number of these trees occur on the limestone itself. In this case the absence of acidity in the soil is probably the cause and not the result of the presence of this forest. The average value of 26 D. cupressinum soils proved to be p H 5.2.

Readings from pure associations of a single species would afford interesting data on the effect of a species in the mass, but with the exception of the N. Menziesii communities stands of this character are rapidly disappearing in the area.

From the above results it appears certain that M. ramiflorus tends to render the soil neutral, while D. cupressinum operates in the reverse direction; N. Menziesii also is undoubtedly an acid-producer in its associations. It is significant that M. ramiflorus soil is considered by some to possess special virtues for agricultural and horticultural purposes.

Leaf Analyses.

Analyses of dried leaves of various species were then carried out and gave the results set out in Table C. Where possible the leaves were collected from specimens growing in one locality, so that any effect derived from the nature of the subsoil would be as uniform as possible.

The results in Table C may explain in some measure the effect of individual species upon the soils around them. Though the figures given above are not likely to be constant for the different species enumerated, in most cases there is a rough correlation between the p H values of the soils and the total ash content of the leaves. The proportion of lime present is, of course, important. The p H value, then, appears to depend in some degree upon the material that each species selects from the soil, some requiring relatively large amounts, others smaller quantities, for their development, this being later on deposited on the ground by fallen leaves and other dead parts.

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Table C.
Species. Locality. Total Ash. Lime content calculated as CaO.
Large-leaved Dicotyledons Melioytus ramiflorus Mt. Flagstaff 13.8 2.7
Myoporum laetum Waikouaiti 10.5 2.2
Fuchsia excorticata Mt. Flagstaff 9.7 1.7
Griselinia littoralis " 5.3 1.5
Conlfers Podocarpus totara Mt. Flagstaff 5.8 2.4
Dacrydium cupressinum " 5.2 2.0
Podocarpus Hallii " 4.8 1.7
Libocedrus Bidwillii " 4.2 1.8
Small-leaved Dicotyledons Nothofagus Menziesii Bethune Gully 3.5 0.9
Leptospermum cricoides Mt. Flagstaff 3.2 0.7
Ground Ferns Blechnum discolor Mt. Flagstaff 9.6 0.6
Polystichum vestitum " 4.1 0.5

(All figures are parts per 100 of dried substances. Leaves dried at 100° C.)

That the soils of M. ramiflorus give readings about the neutral zone is therefore not surprising when the high total ash and lime content of the leaves of this species is considered; similarly the low total ash and lime content of N. Menziesii leaves would be expected to give rise to acid soils, apart altogether from the fact that the slow decomposition of the leaves of this species favours humus accumulation with its attendant superficial acidity. The low lime content in proportion to total ash of Blechnum discolor is striking, the ash appearing to contain a large proportion of silica.

Some other factors to be considered in this connection are (a) rate of decomposition of leaves, the leaves of N. Menziesii and D. cupressinum, for example, being able to resist decomposition for long periods, while those of M. ramiflorus and F. excorticata rapidly disappear from the forest-floor; (b) amount of leaf-matter shed annually by individual species; (c) the effect of a plant's underground parts on the reaction of the soil.

Some Effects of Soil-Reaction on Plants

Evidence concerning the effect of soil-reaction on plants has been steadily accumulating in recent years. Both alkalinity and extreme acidity are injurious to most plants and hydrogen-ion concentration greatly affects soil processes and soil fertility; indeed, it has been shown that soil-reaction is of prime importance as a factor determining the distribution of many species and communities of plants and that probably every species of higher plant has a definite range of soil-reaction which it can tolerate, some with

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quite a narrow range, others with a wide range (Tansley and Chipp, 1926, pp. 123, 129). According to Britton (1929, pp. 519–530) a relatively small change of, for example, 1 to 2 units of p H value is sufficient to retard the growth of certain types of plants and to encourage others to a corresponding degree, and it is important to note that the soil bacteria are even more sensitive than plants to change in soil reaction. The availability of the mineral nutrients of the soil is also influenced by changes in p H value and soil-reaction has a decided effect on the activity of nitrification, this being greatly slowed down when the hydrogen-ion concentration is high. Russell (1932, p. 488), when discussing agricultural crops, divides plants into three groups, according to their behaviour towards soil-reaction :—


Plants tolerating only small variations in soil-reaction, the optimum being at or about the neutral zone or on the alkaline side.


Plants tolerating only small variations in soil-reaction, the optimum being on the acid side.


Plants tolerating considerable variation in soil-reaction.

The foregoing serves to show the advances made in recent years in the study of hydrogen-ion concentration of soils and the importance placed on its effect on plant-life generally.


“A combination of soil and vegetation,” says Jacks (1934, p. 6) “might make a better unit for natural study than either separately, but owing to the different ways in which pedology and ecology have evolved, each since has demanded a distinct training for its more specialised branches, and they are not so closely interwoven as might now be desired.” But the “unit” is a vast complex, and many years of work are ahead before an adequate synthetic treatment is likely to be accomplished. Of recent years one aspect of the problem has received a good deal of attention, the part played by hydrogen-ion concentration in the soil solution. Koslowska (1934, p. 396) sums up: “The general result of these numerous experiments and observations is the following: (1) The existence of a series of species showing the optimum of their development at a certain constant p H value, and (2) the constant dependance of the occurrence of certain plant associations on soil with a constant p H value.” She suggests the need for investigating the “properties of plants themselves on which the development of different concentrations of hydrogen-ions depend.” She concludes from her studies of plants growing in nutrient solutions with different degrees of p H that plants possess the power of changing the reaction of the medium, and that “species occurring in the field within the limits of narrow p H ranges characteristic of particular plant associations exhibit very clearly both the power of alkalising acidic and of acidifying alkaline liquids.”

Studies of succession have made it clear that it is as important to consider the influence of the plant on the habitat as to examine

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the influence of the habitat on the plant-covering, though the multiplicity of factors involved makes the problem an extremely intricate one. Investigations on the power of different species within an association to control or at least to modify the p H value of the soil in their immediate neighbourhood appear to provide a fruitful method of attack. We recognise that the contribution here offered is a slight one, but we consider that the data obtained show that this modifying power of the plant is a very real one.

Müller (as cited by Jacks, 1934, p. 7) as long ago as 1887 found on Danish heaths “that whereas the soil under a heather vegetation was generally strongly leached (the modern podsol), the soil under the occasional oaks that were growing among the heather was often of an entirely different type, showing an almost homogeneous vertical profile with little indication of leaching (the modern ‘brown soil’ or ‘Braunerde’). The two kinds of soil had formed under identical conditions of climate, geology and aspect, and the difference between them must have been caused by the influence of the heather on the one hand, and of the oak on the other.” As a modern example of this type of work may be cited the paper of Raunkiaer (1922). He remarks, “If it can be proved that a species or a formation or a type of formation affects the p H value of the soil in a definite direction, we shall then have demonstrated one of the factors which may be co-determinant when a given formation or type of formation in course of time alters its environment, and by that means brings about its own downfall; it alters in fact the environment in favour of another combination of species.” The averages of some of Raunkiaer's determinations, using the colorimetric method, are here brought together.


Pasture: 6.05—sprucewood (70–80 years old, planted on portions of the pasture) : 4.03;


Pasture: 6.06—beechwood (40–80 years old) : 5.26.


Pasture: 5.91—oakwood (40–50 years old) : 4.87.

Summarizing all his data, Raunkiaer concludes, “Compared with pasture, woodland makes the soil more acid, and the deeper the shade of the wood the greater is the degree of acidity. Further, the soil returns to its original degree of acidity if the wood again gives place to pasture which is allowed to occupy the ground for a long period.” His results support the conclusion we have drawn from our studies that an association may, in time, so modify its own soil reaction that the seedlings of its own members cannot become established within that association. Lindquist (1931, 1. 501) in his work on Scandinavian beechwoods concluded that there is a direct relation between decreasing acidity and an increasing number of beech-seedlings.

We hope that this preliminary report may be an incentive to other workers to join in gathering data on a problem hitherto practically untouched by ecologists in New Zealand, where numerous clearly-marked successions are easily available for investigation.

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We are greatly indebteded to Dr H. H. Allan, Plant Research Station, Palmerston North, for placing at our disposal certain literature and for his unfailing interest in our work at all times. Our thanks are also due to Dr R. Gardner for leaf analyses and to Mr F. R. Meldrum, M.Sc., for the electrometric p H determinations.

Literature Cited.

Britton, H. T. S., 1932. Hydrogen Ions, London.

Jacks, G. V., 1934. Soil, Vegetation and Climate, Imp. Bur. Soil Sci. Techn. Comm., no. 29.

Koslowska, A., 1934. The Influence of Plants on the Concentration of Hydrogen Ions in the Medium, Journ. Ecol., vol. 22, pp. 396–419.

Lindquist, B., 1931. Den Skandinaviska, Bokskogens Biologi, Stockholm.

Raunkiaer, C., 1922. Forskellige Vegetationstypers forskellige Indflydelse paa Jordbundens Surhedsgrad Brintionkoncentration, Kgl. Danske Videnskabernes Selskab, Biol. Medd., p. 515. (Translated as chap. 14 of The Life Forms of Plants and Statistical Plant Geography, Oxford, 1934).

Russell, E. J., 1932. Soil Conditions and Plant Growth, ed. 6, London.

Tansley, A. G., and Chipp, T. F., 1926. Aims and Methods in the Study of Vegetation, London.